244 research outputs found
Chiral nature of magnetic monopoles in artificial spin ice
Micromagnetic properties of monopoles in artificial kagome spin ice systems
are investigated using numerical simulations. We show that micromagnetics
brings additional complexity into the physics of these monopoles that is, by
essence, absent in spin models: besides a fractionalized classical magnetic
charge, monopoles in the artificial kagome ice are chiral at remanence. Our
simulations predict that the chirality of these monopoles can be controlled
without altering their charge state. This chirality breaks the vertex symmetry
and triggers a directional motion of the monopole under an applied magnetic
field. Our results also show that the choice of the geometrical features of the
lattice can be used to turn on and off this chirality, thus allowing the
investigation of chiral and achiral monopoles.Comment: 10 pages, 4 figure
Role of critical spin fluctuations in ultrafast demagnetization of transition-metal rare-earth alloys
Ultrafast magnetization dynamics induced by femtosecond laser pulses have
been measured in ferrimagnetic Co0.8Gd0.2, Co.74Tb.26 and Co.86Tb.14 alloys.
Using element sensitivity of X-ray magnetic circular dichroism at the Co L3, Tb
M5 and Gd M5 edges we evidence that the demagnetization dynamics is element
dependent. We show that a thermalization time as fast as 280 fs is observed for
the rare-earth in the alloy, when the laser excited state temperature is below
the compensation temperature. It is limited to 500 fs when the laser excited
state temperature is below the Curie temperature (Tc). We propose critical spin
fluctuations in the vicinity of TC as the mechanism which reduces the
demagnetization rates of the 4f electrons in transition-metal rare-earth alloys
whereas at any different temperature the limited demagnetization rates could be
avoided.Comment: 11 pages, 4 figure
360 degree domain wall generation in the soft layer of magnetic tunnel junctions
High spatial resolution X-ray photo-emission electron microscopy technique
has been used to study the influence of the dipolar coupling taking place
between the NiFe and the Co ferromagnetic electrodes of micron sized,
elliptical shaped magnetic tunnel junctions. The chemical selectivity of this
technique allows to observe independently the magnetic domain structure in each
ferromagnetic electrode. The combination of this powerful imaging technique
with micromagnetic simulations allows to evidence that a 360 degree domain wall
can be stabilized in the NiFe soft layer. In this letter, we discuss the origin
and the formation conditions of those 360 degree domain walls evidenced
experimentally and numerically
Non-universality of artificial frustrated spin systems
Magnetic frustration effects in artificial kagome arrays of nanomagnets with
out-of-plane magnetization are investigated using Magnetic Force Microscopy and
Monte Carlo simulations. Experimental and theoretical results are compared to
those found for the artificial kagome spin ice, in which the nanomagnets have
in-plane magnetization. In contrast with what has been recently reported, we
demonstrate that long range (i.e. beyond nearest-neighbors) dipolar
interactions between the nanomagnets cannot be neglected when describing the
magnetic configurations observed after demagnetizing the arrays using a field
protocol. As a consequence, there are clear limits to any universality in the
behavior of these two artificial frustrated spin systems. We provide arguments
to explain why these two systems show striking similarities at first sight in
the development of pairwise spin correlations.Comment: 7 pages, 6 figure
Energy levels of interacting curved nano-magnets in a frustrated geometry: increasing accuracy when using finite difference methods
The accuracy of finite difference methods is related to the mesh choice and
cell size. Concerning the micromagnetism of nano-objects, we show here that
discretization issues can drastically affect the symmetry of the problem and
therefore the resulting computed properties of lattices of interacting curved
nanomagnets. In this paper, we detail these effects for the multiaxe kagome
lattice. Using the Oommf finite difference method, we propose an alternative
way of discretizing the nanomagnet shape via a variable moment per cell scheme.
This method is shown to be efficient in reducing discretization effects.Comment: 4 pages, 5 figure
Artificial Kagome Arrays of Nanomagnets: A Frozen Dipolar Spin Ice
Magnetic frustration effects in artificial kagome arrays of nanomagnets are
investigated using x-ray photoemission electron microscopy and Monte Carlo
simulations. Spin configurations of demagnetized networks reveal unambiguous
signatures of long range, dipolar interaction between the nanomagnets. As soon
as the system enters the spin ice manifold, the kagome dipolar spin ice model
captures the observed physics, while the short range kagome spin ice model
fails.Comment: 4 pages, 4 figures, 1 tabl
Element resolved ultrafast demagnetization rates in ferrimagnetic CoDy
Femtosecond laser induced ultrafast magnetization dynamics have been studied
in multisublattice CoxDy1-x alloys. By performing element and time-resolved
X-ray spectroscopy, we distinguish the ultrafast quenching of Co3d and Dy4f
magnetic order when the initial temperatures are below (T=150K) or above
(T=270K) the temperature of magnetic compensation (Tcomp). In accordance with
former element-resolved investigations and theoretical calculations, we observe
different dynamics for Co3d and Dy4f spins. In addition we observe that, for a
given laser fluence, the demagnetization amplitudes and demagnetization times
are not affected by the existence of a temperature of magnetic compensation.
However, our experiment reveals a twofold increase of the ultrafast
demagnetization rates for the Dy sublattice at low temperature. In parallel, we
measure a constant demagnetization rate of the Co3d sublattice above and below
Tcomp. This intriguing difference between the Dy4f and Co3d sublattices calls
for further theoretical and experimental investigations.Comment: 6 Figure, 2 Table
Superconductivity assisted change of the perpendicular magnetic anisotropy in V/MgO/Fe junctions
Distance Properties of Short LDPC Codes and their Impact on the BP, ML and Near-ML Decoding Performance
Parameters of LDPC codes, such as minimum distance, stopping distance,
stopping redundancy, girth of the Tanner graph, and their influence on the
frame error rate performance of the BP, ML and near-ML decoding over a BEC and
an AWGN channel are studied. Both random and structured LDPC codes are
considered. In particular, the BP decoding is applied to the code parity-check
matrices with an increasing number of redundant rows, and the convergence of
the performance to that of the ML decoding is analyzed. A comparison of the
simulated BP, ML, and near-ML performance with the improved theoretical bounds
on the error probability based on the exact weight spectrum coefficients and
the exact stopping size spectrum coefficients is presented. It is observed that
decoding performance very close to the ML decoding performance can be achieved
with a relatively small number of redundant rows for some codes, for both the
BEC and the AWGN channels
Axion searches with the EDELWEISS-II experiment
We present new constraints on the couplings of axions and more generic
axion-like particles using data from the EDELWEISS-II experiment. The EDELWEISS
experiment, located at the Underground Laboratory of Modane, primarily aims at
the direct detection of WIMPs using germanium bolometers. It is also sensitive
to the low-energy electron recoils that would be induced by solar or dark
matter axions. Using a total exposure of up to 448 kg.d, we searched for
axion-induced electron recoils down to 2.5 keV within four scenarios involving
different hypotheses on the origin and couplings of axions. We set a 95% CL
limit on the coupling to photons GeV in
a mass range not fully covered by axion helioscopes. We also constrain the
coupling to electrons, , similar to the more
indirect solar neutrino bound. Finally we place a limit on , where is the
effective axion-nucleon coupling for Fe. Combining these results we
fully exclude the mass range keV for DFSZ axions and
keV for KSVZ axions
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